The present invention relates to a method of producing high-strength steel pipes which consists mainly of a martensitic and/or bainitic microstructure and can be used as high-strength line pipes of API X80 grade or higher. Steel pipes produced by this method are low in yield ratio and high in roundness or circularity in spite of their superior strength.
Those steel pipes, which are currently produced by the UOE process and used in practical pipelines, are up to API X70 grade. The practical use of API X80 grade steel pipes is found only in a few instances in the world. This is because high-strength steel pipes of X80 or higher grade become high in yield ratio and it is difficult to attain a yield ratio not higher than the tolerance limit prescribed in the relevant API specification, and because it is technologically difficult to establish basic characteristics of pipes, including strength, toughness and so forth. Furthermore, for putting steel pipes of X80 or higher grade to practical use, evaluation of the safety of such high-strength steel in actual application to pipelines is required.
However, for improving the conveyance efficiency, it is necessary to improve the strength of line pipes and to perform conveyance under high pressure. In recent years, even high-strength steel pipes of X100 or higher grade have been in demand.
According to the API (American Petroleum Institute), a steel of X60 grade should have a yield strength of 60 ksi (413 MPa) or higher. X80 grade means 80 ksi (551 MPa) or higher, and X100 grade means 100 ksi (689 MPa) or higher. At present, the API specification specifies steels up to X80 grade. The term “high-strength steel pipe”, as used herein, means a steel pipe of X80 or higher.
High-strength steel pipes produced by the UOE process encounter new problems that have not been encountered by low-strength steel pipes. One of them is the increase in yield ratio.
For line pipes, it is prescribed, for providing safety, that the yield ratio, namely the value “(yield strength/tensile strength)×100 (%)”, should be not higher than 93%. Low-strength steel pipes can easily meet this requirement (yield ratio of not higher than 93%). In the case of high-strength steel pipes consisting mainly of martensite and/or bainite, however, it is difficult to secure a yield ratio of not higher than 93%, since the increase in yield strength due to work hardening is significant.
In the UOE process, produced pipes are subjected to the step of expansion. The main objectives of expansion are to adjust the shape and form, typically roundness or circularity, and remove the residual stress resulting from welding. However, this expansion results in an increase in yield strength, hence an increase in yield ratio This tendency is more remarkable in high-strength steel pipes, consisting mainly of a martensitic or bainitic structure, than in low-strength steel pipes, having a ferrite-bainite or ferrite-pearlite structure.
In Laid-open Japanese Patent Application (JP-A) H09-1233 or U.S. Pat. No. 5,794,840, there is disclosed a method of adjusting the characteristics of steel pipes in steel pipe production by the conventional UOE process. The method comprises carrying out cold expansion and cold reduction in combination. However, as is evident from the examples described in the above-cited publication, the target of this method is a pipe of X70 grade. According to claim 2 therein, pipe reduction up to 2% is followed by expansion up to 4% and, according to claim 3, pipe expansion up to 2% is followed by reduction up to 4%.
Among the above methods, the method in which pipe expansion is carried out after reduction, when applied to high-strength steel pipes, causes an increase in yield ratio, leading to failure to meet the above-mentioned requirement (not higher than 93%). As for the method in which the pipe reduction follows expansion, on the other hand, application of such a high degree of pipe expansion as 2% and such a high degree of reduction as 4%, when applied to high-strength steel pipes, results in a marked decrease in the toughness of the steel pipes.
To sum up, the invention disclosed in JP-A H09-1233 or U.S. Pat. No. 5,794,840 is not concerned with a method of producing high-strength steel pipes consisting mainly of a martensitic and/or bainitic microstructure. The publication cited mentions nothing about how to maintain the yield ratio of high-strength steel pipes at low levels or secure the roundness thereof.
The influences of pipe expansion and reduction on the mechanical properties of steel pipes vary depending on the metallographic structure of the pipes. Therefore, the influences of pipe expansion and reduction on low-strength steel pipes having a ferrite-bainite or ferrite-pearlite structure and those on high-strength steel pipes consisting mainly of a martensitic and/or bainitic structure should be studied separately.
At present, there are no findings about a production process in which the problem of the yield ratio of high-strength steel pipes becoming excessively high can be solved. It is an object of the present invention to provide a method of producing steel pipes by which the above-mentioned high yield ratio problem intrinsic in high-strength steel pipes can be solved and, at the same time, the roundness of pipes can be secured.